The case of a 70 year-old man undergoing bilateral carotid endarterectomy (CEA), for whom alterations in his bispectral index (BIS) values were noted during general anesthesia, is presented. Prior to bypass of the internal carotid artery (ICA), there were no significant differences in bilateral BIS values. After bypass of the left ICA, the left BIS increased to approximately 60, while the right BIS remained at 40. Four months later, no such phenomenon was found during a right CEA. BIS is useful in detecting cerebral hypoperfusion during a lower limit of autoregulation. Attaching bilateral BIS monitors may indicate successful reperfusion of cerebral blood flow in CEA.